1. Field of the Invention
The present invention relates to a surge protector device and its fabrication method which returns itself to its non-conductive state in a very short time after conversion to its conductive state by a surge including thunder.
2. Related Background Art
A surge protector device including an arrester is very important device to protect various electronic apparatuses from a surge including thunder. The surge protector device is a general name of apparatuses which are used in order to protect other electronic apparatuses from excess voltage, that is, a surge. An arrester is used to protect other electronic apparatuses from thunder, that is extremely high voltage and large current. The arrester is one of the surge protector apparatuses. The term of “protector device” is used here to indicate apparatuses which are used in order to protect other electronic apparatuses from excess voltage or excess current. However, the excess voltage is not limited to only extremely high voltage such as thunder but includes low voltage if it is excess to a specified voltage.
A glass-tube type arrester has been conventionaly used. It contains special gas between two electrodes in a glass tube. It is non-conductive unless surge is induced. When surge or thunder is induced, discharge starts and the gas between the electrodes changes to conductive. Current passes through the arrester, and it is led to the earth. Discharge does not stop immediately after surge ceases. The arrester cannot protect other electronic apparatuses from continuous current or next attack by surge or thunder. There were serious problems in a glass-tube and other type protector devices which have been used. One of the problems is that a protector device must change from its resistive state to a conductive state in a very short time such as 0.03 μsec. when it is attacked by surge. Another problem is that a protector device should return from the conductive state to the original resistive state when surge ceases.
In order to solve these problems in the prior art, an improved arrester was proposed (Japanese Patent Publication No. 118361/1995, “Molybdenum Arrester” by Seita Ohmori). It is what uses a plurality of molybdenum bars whose surface was oxidized. This arrester will be called here as a “molybdenum arrester”.
The molybdenum arrester leads current to the earth when surge or thunder is induced. The molybdenum arrester is very useful and economically efficient because it repeats the change between the conductive and non-conductive states automatically.
It is possible to use metals other than molybdenum in the protector device which functions with the same principle as the molybdenum arrester. Tantalum, chromium and aluminum are included in such metals.
There is a serious problem in the improved protector device by Ohmori which results from the fact that the protector device uses a simple pileup of a plurality of bars which have resistive films on their surfaces. FIG. 1 shows schematically the arrester (10) of the prior art which is called the molybdenum arrestor proposed by Ohmori (Japanese Patent Publication No. 118361/1995 “Molybdenum Arrester”).
The arrester (10) includes two molybdenum bars (11) which have high resistive oxide films (12) on their surfaces and electrodes (13). The arrestor (10) uses the breakdown phenomenon at the interface between the high resistive films (12). A breakdown voltage depends largely on microscopic structure of the interface. That is, as shown in FIG. 2, the high resistive films (12) on the two molybdenum bars come in contact with each other point by point microscopically although they seem to contact line by line or surface by surface macroscopically.
There exists a layer (21) of air with a thickness of at least several atomic sizes between the high resistive films on the two molybdenum bars. The breakdown is what occurs in this layer of air. Therefore, an oscillator of voltage is observed as shown in FIG. 4 with an oscilloscope when an direct voltage is applied to the arrestor as shown in FIG. 1 which was proposed by Ohmori through an circuit (30) shown in FIG. 3. In FIG. 3, the circuit (30) includes a power source (31), a sample (32), resistors (33, 34), an oscilloscope (35), and an amperameter (36). Similarly, a very sharp pulse of current is observed when an alternating voltage is applied to the Ohmori's arrestor. These phenomena mean that the Ohmori's arrestor cannot be used in practical uses. There has been no report of test on Ohmori's arrestor as described above by Ohmori and other peoples. The fact described above mean that it is impossible to realize a practically useful arrestor as far as it is composed of molybdenum bars simply piled up. In other words, it is impossible to realize a practically useful surge protector device as long as it uses breakdown phenomena in a layer of air between two surfaces.
It is desirable, therefore, to provide a surge protector device which does not use breakdown phenomena in a layer of air between two surfaces.